Face detection and comparision

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HI, I need to detect a face from an image and need to compare to the faces which i m having and check whether they are same or not....I m new to opencv so i dont have any idea..i tried and able to identify the face from the image and had a problem to compare the obtained image from the other images so any one please help in thiss.... i am trying to in c++

this is the below code i tried

#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>

using namespace std;
using namespace cv;

// Function Headers
void detectAndDisplay(Mat frame);

// Global variables
// Copy this file from opencv/data/haarscascades to target folder
string face_cascade_name = "C:\\opencv\\data\\haarcascades\\haarcascade_frontalface_alt.xml";
CascadeClassifier face_cascade;
string window_name = "Capture - Face detection";
int filenumber; // Number of file to be saved
string filename;

// Function main
int main(void)
{
    // Load the cascades
    if (!face_cascade.load(face_cascade_name))
    {
        printf("--(!)Error loading\n");
        return (-1);
    };

    // Read the image file
    Mat frame = imread("images.jpg");

    for (;;)
    {
        // Apply the classifier to the frame
        if (!frame.empty())
        {
            detectAndDisplay(frame);
        }
        else
        {
            printf(" --(!) No captured frame -- Break!");
            break;
        }

        int c = waitKey(10);

        if (27 == char(c))
        {
            break;
        }
    }

    return 0;
}

// Function detectAndDisplay
void detectAndDisplay(Mat frame)
{
    std::vector<Rect> faces;
    Mat frame_gray;
    Mat crop;
    Mat res;
    Mat gray;
    string text;
    stringstream sstm;

    cvtColor(frame, frame_gray, COLOR_BGR2GRAY);
    equalizeHist(frame_gray, frame_gray);

// Detect faces
    face_cascade.detectMultiScale(frame_gray, faces, 1.1, 2, 0 | CASCADE_SCALE_IMAGE, Size(30, 30));

// Set Region of Interest
    cv::Rect roi_b;
    cv::Rect roi_c;

    size_t ic = 0; // ic is index of current element
    int ac = 0; // ac is area of current element

    size_t ib = 0; // ib is index of biggest element
    int ab = 0; // ab is area of biggest element

    for (ic = 0; ic < faces.size(); ic++) // Iterate through all current elements (detected faces)

    {
        roi_c.x = faces[ic].x;
        roi_c.y = faces[ic].y;
        roi_c.width = (faces[ic].width);
        roi_c.height = (faces[ic].height);

        ac = roi_c.width * roi_c.height; // Get the area of current element (detected face)

        roi_b.x = faces[ib].x;
        roi_b.y = faces[ib].y;
        roi_b.width = (faces[ib].width);
        roi_b.height = (faces[ib].height);

        ab = roi_b.width * roi_b.height; // Get the area of biggest element, at beginning it is same as "current" element

        if (ac > ab)
        {
            ib = ic;
            roi_b.x = faces[ib].x;
            roi_b.y = faces[ib].y;
            roi_b.width = (faces[ib].width);
            roi_b.height = (faces[ib].height);
        }

        crop = frame(roi_b);
        resize(crop, res, Size(128, 128), 0, 0, INTER_LINEAR); // This will be needed later while saving images
        cvtColor(crop, gray, CV_BGR2GRAY); // Convert cropped image to Grayscale

        // Form a filename
        filename = "";
        stringstream ssfn;
        ssfn << filenumber << ".png";
        filename = ssfn.str();
        filenumber++;

        imwrite(filename, gray);

        Point pt1(faces[ic].x, faces[ic].y); // Display detected faces on main window - live stream from camera
        Point pt2((faces[ic].x + faces[ic].height), (faces[ic].y + faces[ic].width));
        rectangle(frame, pt1, pt2, Scalar(0, 255, 0), 2, 8, 0);
    }

// Show image ...